The eventual commercial production of shale oil in sufficient quantities to constitute a significant replacement of petroleum oil will involve the handling of enormous quantities of inert inorganic mineral refuse in the process of recovering the kerogen content from the oil shale. For example, commercially recoverable oil shale generally contains from about 85 percent to about 95 percent mineral matter, with the kerogen-rich material constituting a very minor proportion of the overall in-place oil shale.
Ordinarily, the first step in any oil shale process is to reduce the bulk material to a size that can be effectively processed. In particular, once it has been crushed to a size amenable for easier handling, the shale can be further ground to expose or begin to separate the desired kerogen found in the mineral matrix. Fine grinding is more particularly desirable when the shale is subjected to a beneficiation process to increase significantly the percentage of kerogen in the feed and remove most of the kerogen-poor mineral gangue. As a result, an economic process which significantly reduces the amount of oil shale which must be handled and treated to yield a given amount of kerogen and which significantly reduces the amount of polluting shale waste would be advantageous.
Various oil shale beneficiating procedures have been proposed. Those separations most proposed are predicated on the differential occurrence of kerogen in the various lumps, pieces and particles of oil shale following the various methods of size reduction and comminution. Since the larger pieces in a reduced shale tend to have a higher kerogen content, simple screening can effect a beneficiation, as described in U.S. Pat. No. 3,133,010. Since kerogen-rich particles possess a lower specific gravity, gravity separation in a dense liquid can also effect a moderate separation, as also mentioned in the reference above. Since kerogen-rich particles differ in wettability from kerogen-poor particles, separation in an aqueous medium by froth flotation is also a significant means of segregating kerogen from the inorganic mineral matter, one method of which is described in U.S. Pat. No. 3,973,734. Two particular beneficiating processes deriving significant advantage from the present invention are claimed in U.S. Patent Applications entitled "Process, for Beneficiating Oil Shale Using Froth Flotation", Ser. No. 768,901 Datta et al., and "Process for Beneficiating Oil Shale Using Selective Flocculation", Ser. No. 769,188 Datta et al., both filed contemporaneously herewith.
The intimate nature of the mineral-matter/kerogen in oil shale requires a complex size reduction procedure. Because of its anisotropy, i.e., having different properties in different axes, shale on comminution results in a coarser kerogen-rich fraction which is more resistant to grinding, and a kerogen-poor fraction. This selective fractionation of the oil shale may also be utilized profitably during its beneficiation.